How do I optimise storage?

It is a process of moving data such as photos to iCloud and removing them from the drive to free up space. Personally, I'm not a fan of the process. I just use another drive and move what I must from the startup drive to the second drive. But at this point, if you don't have 8GBs of free space on your drive, then you are already in trouble. See the following:

How to Free Up Space on The Hard Drive

1. You can remove data from your Home folder except for the /Home/Library/ folder.
2. Visit The XLab FAQs and read the FAQ on freeing up space on your hard drive.
3. Also, see Freeing space on your Mac OS X startup disk.
4. Free up storage space on your Mac.
5. See Where did my Disk Space go?.
6. Be sure to Empty the Trash to recover the space.
7. Replace the drive with a larger one. Check out OWC for drives, tutorials, and toolkits.
8. Use OmniDiskSweeper or GrandPerspective to search your drive for large files and where they are located.

If you allow the free space on your drive to drop below 3GBs, then you are in danger of a major directory corruption that will render the drive unusable until it is completely erased. This means you will lose everything on the drive. You should allow at least 10-15GBs of free space at all times. If you need 8GBs for an installer then you need 10GBs of free space, 8GBs for the installer, and another 8GBs during the installation. That means you may need a minimum of 26GBs of free disk space to install an OS X upgrade.

You can move some rarely accessed documents to an external HDD, flash drive, or cloud service (Google Drive gives you 15 GB for free). Alternatively, you can uninstall some apps you don’t use.